1. Field of the Invention
The invention relates to a drive package for acoustic transmitters Current acoustic devices are able to operate both as transmitters and receivers, that is, as measuring transducers for acoustic signals. An acoustic device in which the invention may be used to great advantage is as a so-called Sonar, that is, a transmitter which sends out sound waves under water which, after reflection, can be monitored by hydrophones of various kinds.
2. The Prior Art
It is a well-known fact that low-frequency sound waves can travel longer distances through water than can high-frequency sound waves. For a long time there has also been a considerable need for powerful low-frequency sound transmitters which are capable of working under water, both from a military point of view and from the point of view of the offshore oil and gas industry. Transmitters of various designs for these purposes and fields of use have been available on the market for quite a long time. A summary of such acoustic transmitters is given in an article in DEFENSE SYSTEM REVIEW, November 1984, pages 50-55 entitled "Sonar Transducer Design Incorporates Rare Earth Alloy".
Most acoustic transmitters which are used at present are based on either the piezoelectric effect or on magnetostriction. As is well-known, the piezoelectric effect means that a crystalline substance is subjected to a change in length when an electric voltage is applied to its end surfaces and that a voltage is obtained when the substance is subjected to a physical deformation, respectively. The magnetostriction means that a magnetic material which is subjected to a change of the magnetic flux suffers a change in length and that an externally caused change in length gives rise to a change in the magnetic flux, respectively. This means that a transmitter which utilizes these effects can also, in principle, be used as a receiver.
A variety of different embodiments of acoustic transmitters exist. In low-frequency applications it is common that they have a cylindrical shape with either a circular or elliptical cross section area.
The greatest problem with this type of transmitter is to achieve a sufficiently great amplitude of the oscillations. To this end, either a large transmitter area or a small transmitter area with great amplitude of oscillation would be required.
The introduction of the so-called giant magnetostrictive materials has improved the conditions for obtaining good acoustic transmitters. With such materials as driving elements, amplitude changes may be obtained which may largely amount to 100 times the corresponding changes using piezoelectric materials or using ordinary magnetic materials. Transmitters which utilize these giant magnetostrictive materials have existed on the market for several years.
A frequently occurring embodiment for the actual driving will be described in greater detail starting from a cylindrical transmitter with an elliptical cross section. The cylindrical envelope surface consists of an elastic diaphragm or shell. Inside and parallel to the axis of the cylinder and making contact with the shell are two rods applying pressure to the shell. The cross sectional area of the rods is symmetrically mirror-inverted in relation to the minor axis of the elliptical shell and each rod is delimited by that part of the shell which faces the end of the major axis and a chord parallel to the minor axis. Between the rods and making contact with their plane-parallel sides there is arranged an electrically controlled driving element in the form of a driving rod. The longitudinal axis of the driving rod coincides with the major axis of the elliptically formed cross section and lies midway between the end surfaces of the transmitter. In those case where the magnetostrictive effect is utilized, the driving rod consists of a magnetic material, suitably a giant magnetostrictive material, which with a surrounding winding is magnetized to keep pace with the desired frequency of the transmitter. If the piezoelectric effect is to be utilized, the driving rod is made of a piezoelectric material. The driving rod may, of course, consist in its entirety, or in certain parts, of a material with the desired possibilities of changing the length.
The fundamental embodiment of an acoustic transmitter described above may be different as regards the actual details. An acoustic transmitter with a cylindrical shape and with an elliptical cross section area and with driving rods of a giant magnetostrictive material is disclosed, inter alia, in U.S. Pat. No. 4,901,293 entitled "A Rare Earth Flextensional Transducer".
Swedish published patent application 8901905-3, entitled "Device in acoustic transmitters", also describes a cylindrical transmitter with elliptical cross section. The driving element consists of a body with oppositely located recesses into which pressure rods are inserted. The driving rods, in turn, are fixed into pressure rods which in the same way as above influence the diaphragm.